Highly accurate equilibrium structure of the C2h symmetric N1‐to‐O2 hydrogen‐bonded uracil‐dimer

Abstract

The highly accurate ab initio equilibrium geometry of the hydrogen‐bonded uracil dimer is derived using a composite geometry extrapolation scheme based on all‐electron, complete basis set extrapolated Møller–Plesset perturbation theory using the jun‐pwCV[T,Q]Z basis sets combined with a valence CCSD(T)/cc‐pVTZ high‐level correction. Geometrical changes on dimerization are discussed and the performance of the several density functional approximations (among others SCAN, ωB97M‐V, DSD‐PBEP86‐D3(BJ), and DSD‐PBEP86‐NL) is evaluated. Orbital‐optimized MP2.5 is discussed as a reduced‐cost alternative to the CCSD(T) gradient in the composite scheme. A new reference interaction energy is calculated with explicitly correlated F12‐CCSD(T).

Validating additive correction schemes against gradient‐based extrapolations

“…One of the main arguments for using additive correction schemes, as opposed to the more rigorous extrapolation of energies/gradients/Hessians, is reported to be the decreased computational cost of such corrections . However, the most time‐consuming component of, for example, the “cheap” method is the computation of the CCSD(T)/VTZ component . For frequencies, the cost of the largest‐basis CCSD(T) component will dominate the overall computational scaling—it is therefore unclear how an extrapolation method consisting of the same components can be significantly more expensive than a correction scheme.…”

“…With increasing availability of computational power, programming tricks, clever choice of basis sets, and various approximations to canonical coupled cluster (CC) methods (eg, by density fitting [DF], reduction of virtual space, or local correlation with paired orbitals among others), it is now possible to carry out calculations of (nearly) CC through pertubative triplets (CCSD[T]) quality almost routinely. Coupled to correlation consistent basis sets and extrapolation procedures, one can now obtain highly accurate “gold‐standard” results even for rather large systems …”

“…Perhaps this is why the use of empirical, additive correction schemes to obtain structural parameters, and even properties of molecules has been recently gaining traction. The application of such correction schemes to geometries of increasingly larger systems is becoming more common . The approach has been also applied to harmonic frequencies, as well as dipole moments .…”

“…In short, for geometries, this approach consists of optimizing the structures at each level of theory in the “recipe” individually, and then combining the structural parameters (radii, angles, dihedrals) directly to obtain a “best” composite estimate. In analogy with the above energy scheme, an equivalent geometry‐based correction scheme would be where r x corresponds to an extrapolated value of the parameter (radius), either by optimizing the extrapolated gradient or more commonly by extrapolating the radii directly . The angles, dihedrals, as well as harmonic frequencies can be obtained analogously.…”

Kinetics and Thermodynamics of Reactions Involving Criegee Intermediates: An Assessment of Density Functional Theory and Ab Initio Methods Through Comparison with CCSDT(Q)/CBS Data

“…Nevertheless, we opted on using the DSD-PBEP86-D3BJ functional, which should in principle be more reliable since the underlying exchange-correlation functional includes both exact Hartree-Fock exchange and MP2 correlation energies. 55,56,57,58 Finally, we note that in order to enable a rigorous comparison between the benchmark W3lite-F12 and the DFT results, all the DFT calculations are carried out on the DSD-PBEP86-D3BJ/Def2-TZVPP optimized geometries that were used in the W3lite-F12 calculations.…”